The present invention relates to medical imaging cameras, and more specifically to imaging systems which include a mechanism for determining emission attenuation to compensate emission images for varying patient densities.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a compound labeled with a radiopharmaceutical tracer (isotope) into the body of a patient to be examined. The radiopharmaceutical is a substance that emits photons of gamma radiation at one or more specific energy levels, e.g. 140 keV. By choosing a compound that will accumulate in an organ to be imaged, the compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to that organ of interest. Thus measuring the intensity of the gamma rays emitted from the patient, enables identification of organ characteristics and irregularities.
A planar gamma camera is used to measure the intensity of the photon emission. That camera comprises a stand that supports a detector which includes a collimator, scintillation crystal and a two dimensional array of photomultiplier tubes (PMTs) adjacent to the patient. The collimator is formed by a closely packed array of holes in a lead block, which define preferred photon paths. For example the Optima Low energy, high resolution collimator has over 30,000 holes with each one about 32 mm long and 1.8 mm across with 0.2 mm of lead between the adjacent holes. The collimator preferably allows gamma rays, which pass along the holes, to impinge on a scintillator and attenuates gamma rays which cross the lead septa. The scintillation crystal abuts the collimator on a side opposite the patient to absorb photons and emit light each time a photon is absorbed. The intensity of the light corresponds to the energy level of the photon which causes the light emission. The PMTs detect the light emitted by the scintillation crystal and respond by generating analog electrical intensity signals corresponding to the energy level of each photon.
A processor receives the PMT signals and digitally processes those having energy levels that are associated with photons emitted by the radiopharmaceutical tracer. The processor digitally stores emission information as a two dimensional array of pixels. The processor uses the pixel information to form an emission projection image, known as a view, at the particular camera position.
Most gamma camera systems generate a plurality of emission views, each taken by positioning the gamma camera parallel to and at a different angle about a rotation axis passing through the patient. The angle is incremented between views so that the plurality of projection images can be acquired and used together to construct pictures of transaxial slices of the body using algorithms and iterative methods that are well known to those skilled in the tomographic arts.
Unfortunately, because different materials are characterized by different attenuation coefficients, photons are attenuated to varying degrees as they pass through various portions of a patient's body. For example, a given thickness of bone typically attenuates a greater percentage of photons than the same thickness of tissue. The organ image is degraded when the different anatomical features attenuate the radiation leaving the body, in effect casting shadows on the projection image. When the projection images taken at many different view angles are used to reconstruct a tomographic picture, these shadows create artifacts in the reconstructed picture.
To compensate for such artifacts, a transmission measurement may be made by placing a calibrated radiation source on the opposite side of the patient from the gamma camera and measuring the amount of radiation that passes through the patient. This provides a measurement of how much of the known radiation is attenuated. The transmission radiation source emits gamma rays at an energy level (e.g. 100 keV), distinguishable from the radiopharmaceutical energy level (e.g. 140 keV), thus allowing the gamma camera to separately measure the emission received from each source. However, when imaging a patient using a radiopharmaceutical tracer that has a higher energy than-the transmission source, scattered photons from the tracer can be reduced in energy and mistaken for transmission photons. This "crosstalk" contaminates the transmission image.
A previous approach that reduced the crosstalk effects involved "masking"--electronically defining an acceptance window in the camera's field of view. Radiological events outside the acceptance window were masked, or rejected, from further processing. The acceptance window moves across the field of view in correspondence with a scanning transmission source so that the window instantaneously exposes the area on the cameras detector where transmission events are expected to occur and an electronic mask covers other detector areas. Although this masking reduces the crosstalk, it does not eliminate the need for some crosstalk correction.
The simplest way to acquire the needed crosstalk data is to first perform a complete tomographic acquisition rotation during which only emission data (i.e. photons at energy levels from the radiopharmaceutical tracer) are accepted. During this time an image also can be acquired using the transmission energy levels, but with the transmission gamma source inactive. This latter image provides a measurement of the activity that scatters from the tracer and appears in the transmission energy range which then is used to correct crosstalk in the transmission image. A second acquisition rotation is performed during which the transmission source is swept across the detector field of view at each view angle. The transmission events within the transmission energy range and within the mask acceptance range are detected. While this technique provides a simple process for eliminating crosstalk, it requires two rotations of the detector assembly and prolongs the patient's stay in the imaging system. In addition, the method is susceptible to errors resulting from movement of the patient between the two acquisitions.